When fish stop eating, gasp at the surface, or develop white spots, cottony growth or swelling, first check water quality and equipment, then review recent changes and observe the symptoms. Similar appearances can have different causes; testing the water helps identify environmental problems, but normal readings alone can't rule out infection.[1][2]
What to do first when something seems wrong
If all the fish in a tank suddenly start breathing rapidly, immediately check that the air pump, filter and water flow are working, improve aeration, and at the same time measure water temperature, pH, ammonia and nitrite; if you have a dissolved oxygen meter, take a reading on the spot too. Don't delay dealing with an obvious oxygen shortage or equipment failure while you wait for all the test results.[2]
First record when the problem appeared, how many fish are affected and anything you've done recently. If it won't delay emergency care, take a separate tank water sample before changing the water; fish keep breathing and excreting during transport, so the water in a shipping bag doesn't represent the original tank's water quality.[3]
If several fish die in succession, fish clearly lose their balance, or gill lesions or ulcers spread quickly, contact an aquarium fish veterinarian or fish disease diagnostic lab at the same time. Confirm the cause first, then discuss whether to medicate; treating without a full diagnosis often leads to misjudgment and inappropriate treatment that may actually harm the fish, and medications can also affect the filtration system.[1][3]
Step 1: Test the water and review recent changes
| What to check | Key points |
|---|---|
| Water temperature and pH | Compare with conditions suited to the species; also needed to interpret ammonia toxicity. Watch for heater failure and changes before and after water changes. |
| Ammonia and nitrite | A stable system should aim for levels a test kit can't detect. Check the units of ammonia readings, and interpret them together with water temperature and pH. |
| Dissolved oxygen and equipment | Gasping at the surface isn't only a sign of low oxygen; it can also mean nitrite poisoning or gill disease. Check the air pump, filter and water flow. |
| New water and recent handling | If illness followed a water change, check whether dechlorination was complete and the temperature and pH differences; if you've used medication, monitor water quality more closely. |
Table compiled from resources on aquarium fish management, environmental diseases and ammonia.[1][2][4] If water quality is abnormal, see "Ammonia and Nitrite Poisoning" for how to handle it and how to calculate ammonia. Before a large water change, make sure the new water is suitable; when total ammonia is very high (the Merck Veterinary Manual gives above 5 mg/L as an example) and the water is acidic (pH below 7), a sudden pH rise from the water change may convert ammonium ions into the more toxic un-ionized ammonia.[2]
Also think back to whether you've recently added new fish, suddenly increased feeding, cleaned filter media, had a power outage, transported fish or changed water. When all the fish in a tank become abnormal within a short time, environmental problems should be investigated first; deaths that increase day by day may also be related to infection. The pattern of deaths only provides clues and can't confirm a diagnosis on its own.[3]
Step 2: Narrow it down by symptoms
| Symptom you see | Directions to consider | Key points to confirm |
|---|---|---|
| Several fish gasping at the surface and breathing rapidly | Low oxygen, ammonia or nitrite, gill infection | Test the water and check equipment first; normal water quality doesn't rule out gill disease. |
| Salt-grain-like white spots | Freshwater ich and other similar lesions | Requires microscopy; with gill-only infections, white spots may not be visible. |
| Fine golden or rust-colored dust | Freshwater velvet disease | May not be obvious; the skin, fins or gills need to be examined. |
| Eroded fins or cottony lesions on the mouth | Columnaris, other bacterial infections, water mold | Appearance can't reliably identify the pathogen; mixed infections are possible. |
| Grayish-white cottony fuzz on wounds | Water mold and other attached organisms | Use microscopy and find the original wound or disease. |
| Abdominal swelling and raised scales | Fluid buildup, edema, infection or internal disease | A swollen belly isn't necessarily dropsy; a veterinary exam is needed. |
| Thread-like parasites hanging from the body, or moving discs | Anchor worms, fish lice | After identifying the adults, larvae and eggs in the environment still need to be assessed. |
| Pits on the head or lateral line | Hole-in-the-head disease or other erosive lesions | The species and lesion need to be considered; don't assume flagellates or nutritional deficiency. |
| Floating belly-up or persistently head-down | Buoyancy disorder, internal disease or other causes of imbalance | A veterinarian can use imaging (such as X-rays or ultrasound) to assess internal structures. |
These are directions for differential diagnosis, not a medication chart. The related articles explain each symptom in detail; if fish are breathing rapidly or their condition is deteriorating quickly, seek veterinary care as soon as possible.[1][5]
Prepare sick fish and separate water samples before submission
First call to confirm how samples are received, how many fish are needed and the cost. Live fish showing typical signs are usually the most useful for diagnosis; but if a fish is already very weak or dying, ask the diagnostic lab whether it can be shipped alive. For group outbreaks, the lab may need several representative sick fish. If you keep only one or two pet fish, you can consult a veterinarian first; there's no need to sacrifice fish just to make up the numbers.[3]
If a fish has already died, tell the diagnostic lab right away when it died and how it has been stored, and refrigerate it for submission as instructed. Different tests have different freshness requirements, so a single cutoff in hours can't decide whether a specimen is still useful. In general, don't freeze the fish yourself, and don't let it touch ice or ice packs directly.[1][3]
Pack water samples separately from the fish. You can start by preparing at least 500 mL of original tank water[1]; UF/IFAS's submission guidelines give a clean container of about 1 quart (about 950 mL) as an example[3]. Label the sampling time and the tank, and keep the sample refrigerated and out of the light; the final volume and sampling method depend on the receiving lab. Water temperature, dissolved oxygen and other parameters that change easily should be measured and recorded on site whenever possible.[3]
Include the species, number of fish kept, illness and death records, water quality data, water change and feeding details, and the names, doses and timing of all medications used. This information helps determine whether water quality and pathogens are causing the problem together.
Fish veterinarians and diagnostic services: examples from Taiwan
In Taiwan, the Directory of Ornamental Fish Veterinarians from the Ministry of Agriculture's Veterinary Research Institute can be a starting point for finding a clinic; the directory was compiled in 2021, so call first to confirm whether a clinic currently sees fish. The animal disease diagnostic centers at National Chung Hsing University and National Chiayi University also publish their aquatic animal testing services and fees.[6][7][8]
Group submission services at diagnostic centers differ from individual pet care at animal hospitals. Explain the species, number of fish and the problem first, confirm whether they can take the case, and ask whether basic exams, pathology, culture and molecular tests are billed separately.
Frequently asked questions
If the water looks clear, can I rule out water quality problems?
No. Changes in ammonia, nitrite and dissolved oxygen may not be visible; you need to test and also look at past records, because a single normal reading can't rule out damage from earlier exposure.[2][4]
Could it still be parasites if I don't see any white spots?
It's possible. Some parasites mainly damage the gills, and there may be no obvious lesions on the outside. If breathing is rapid and water quality doesn't explain it, have a professional examine the fish.[5]
Should I move a sick fish to another tank right away?
Ask a veterinarian or diagnostic lab first whether the sick fish needs to be moved. If it's moved into a newly set up, immature system, ammonia may rise during the first two to three weeks after setup, so check the new system's water quality and aeration first.[2] Also, don't assume that removing one fish makes the pathogen disappear from the original tank.
If medication is needed, use a legally registered veterinary drug that's appropriate for the case, following the label and your veterinarian's instructions.
Further reading
- Ammonia and Nitrite Poisoning – Fish Gasping at the Surface and How to Read Water Test Kits
- How to Quarantine New Fish – Isolating for at Least 30 Days and Acclimating Fish to the Tank
- White Spot Disease (Ich) in Freshwater Fish – The Ichthyophthirius Life Cycle and Why It Keeps Coming Back
- Velvet Disease in Freshwater Fish – Gold-Dust Symptoms and How It Differs from Ich
- Columnaris Disease Symptoms – Recognizing Rotting Gills and Fins and Cottony Growth on the Mouth
- Is White Fuzz on Your Fish Water Mold? – Wound Infections and Why Fish Eggs Grow Fuzz
- Causes of Dropsy and Raised Scales in Fish – How to Assess a Swollen Belly and Pinecone Scales
- Parasites Visible on Your Fish – Identifying Anchor Worms and Fish Lice and Understanding Their Life Cycles
- What Causes Hole-in-the-Head Disease – The Evidence on Activated Carbon, Flagellates and Nutrition
- Goldfish Floating Upside Down or Head-Down – Causes of Buoyancy Disorders and When to See a Vet
References
- Petty BD, Francis-Floyd R, Yanong RPE. Management of Aquarium Fish. Merck Veterinary Manual, revised 2022.
- Francis-Floyd R, Yanong RPE, Petty BD. Environmental Diseases of Aquatic Animals in Aquatic Systems. Merck Veterinary Manual, revised 2023.
- Stilwell NK, Yanong RPE, Francis-Floyd R. Submission of Fish for Diagnostic Evaluation. UF/IFAS, FA055, 2001 (revised 2025).
- Francis-Floyd R, Watson C, Petty D, Pouder DB. Ammonia in Aquatic Systems. UF/IFAS, FA031, 1990 (revised 2022).
- Petty BD, Francis-Floyd R, Yanong RPE. Parasitic Diseases of Fish. Merck Veterinary Manual, revised 2022.
- Veterinary Research Institute, Ministry of Agriculture. Directory of Ornamental Fish Veterinarians, 2021.
- National Chung Hsing University Animal Disease Diagnostic Center. Fee schedule, 2026 revision, accessed October 2026.
- National Chiayi University Animal Disease Diagnostic Center. Fee schedule, 2025 revision, accessed October 2026.